The invention relates to a liquid-crystal display (LCD) device comprising a liquid-crystal cell having a twisted nematic liquid-crystalline material which is arranged between two substrates having electrodes and between two polarizers, and more particularly relates to such a device in which a layer of an optically anisotropic material is provided between the polarizers.
If desirable, the layer of optically anistropic material may be a self-supporting birefringent foil. Such a display device is described in European Patent Application 0 246 842.
Chirality in liquid-crystalline materials leads to rotation of the molecules in a direction perpendicularly to their longitudinal axis. In the case of liquid-crystalline materials in the so-called cholesteric phase, the pitch of the rotation is 0.1 to 1 .mu.m. For application in, for example, datagraphic displays using multiplex drive, a larger pitch of the order of magnitude of the cell thickness of the display device is desirable. Such a pitch is obtained by adding a chiral liquid-crystalline compound as a dopant to a nematic liquid crystal. With such materials, supertwisted nematic (STN) liquid-crystal display devices are manufactured, the total twist of the molecular axis across the cell being, for example, between 180.degree. and 270.degree.. Such display devices have the disadvantage that the optical properties depend to a large extent on the wavelength, of the light so that a high contrast and a colourless image (black/white instead of e.g., blue/yellow) is difficult to attain. Said disadvantage can be overcome in a known manner by using a combination of two identical cells, one of which contains left-handed liquid-crystalline material and the other contains right-handed liquid-crystalline material. When the molecular axis at the front of the second cell extends perpendicularly to the molecular axis at the rear of the first cell the wavelength dependence of the optical properties is completely compensated. However, as a result of this second cell the liquid-crystal display device becomes heavier and less compact. According to a simpler alternative, the second cell is replaced by a uniaxial foil having an adapted birefringence. In this case, the compensation of wavelengths dependence is not complete, resulting in the display device exhibiting a contrast reduction and a certain degree of colour in the voltageless state. Another alternative consists in the use of a twisted stack of uniaxial foils. This solution gets closer to the ideal situation (a twist and a birefringence which are equal to the twist and birefringence of a supertwisted nematic liquid-crystal display device) as the number of foils increases. However, this leads to a considerably more complicated production process. Instead of a foil, it is alternatively possible to use a birefringent layer on a suitable substrate. In European Patent Application 1-0 007 574 a description is given of liquid-crystalline polymer materials having a chiral dopant in the form of a copolymerizable monomer. Such polymer materials are linear and have side groups which exhibit liquid-crystalline properties. A thin layer is manufactured from a solution or a melt and is oriented in the rubbery liquid-crystalline state, after which it is cooled to a temperature below the glass transition temperature. Such layers are often turbid owing to local fluctuations in the refractive index caused by a poor order. Moreover, heating above the glass transition temperature, executed only once, leads to a permanent loss of order. Besides, the method does not permit the pitch and the thickness of the polymer layer to be accurately adjusted.
One of the objects of the invention is to provide a liquid-crystal display device and a birefringent foil or layer of optically anisotropic material for use in such a display device, the birefringent foil or layer being optically clear and having a large temperature resistance. Another object of the invention is to provide a supertwisted nematic liquid-crystal display device having a high contrast, the voltageless state being substantially completely dark and colourless, and the voltage on state being highly transparent.
A further object of the invention is to provide a foil or an anisotropic layer which can be manufactured with the desired accuracy in a simple manner.
Another object of the invention is to provide a material which can suitably be used in the birefringent foil or layer.
According to the invention, these objects are achieved by a liquid-crystal display device as described in the opening paragraph, characterized in that the birefringent foil or layer of optically anisotropic material is formed from a synthetic resin composition which comprises a polymer network having a helicoidal order. The synthetic resin composition is preferably manufactured from a curable liquid-crystalline composition having a chiral dopant.
In a preferred embodiment of the display device according to the invention, the synthetic resin composition is formed by curing liquid-crystalline monomers or oligomers which consist of compounds with two or more acrylate-ester groups. Instead of acrylate compounds, epoxides, vinyl ethers and thiolene compounds can alternatively and satisfactorily be used as liquid-crystalline monomers.
An ordered synthetic resin composition can be obtained, for example, by orienting a liquid-crystalline monomer and freezing said orientation by exposure to UV light in the presence of a light-sensitive initiator.
A chiral dopant, for example, a compound with an asymmetrically substituted carbon atom is added to the monomer. This dopant brings about a rotation of the monomer molecules in a direction perpendicular to the longitudinal axis of the molecules. By arranging the monomer between two polyimide-coated and rubbed surfaces or other orienting surfaces such as obliquely deposited SiO, the degree of rotation can be adjusted as a function of the natural pitch (the pitch without the presence of such surfaces), the distance between the rubbed surfaces and the direction of rubbing of the surfaces. Subsequently, the rotation in the still liquid monomer composition is fixed by polymerization of the reactive end groups under the influence of UV light or irradiation using electrons. The desired order is rapidly obtained and is substantially perfect, so that a clear film or thin layer is attained. As a result of the use of monomers having at least two functional groups an ordered polymer network is maintained up to very high temperatures.
Preferably, a curable composition is used having liquid-crystalline monomers or oligomers which consist of compounds with two or more acrylate-ester groups. If desirable, the curable synthetic resin composition may comprise a mixture of various oligomeric compounds. Besides, the synthetic resin composition may comprise one or more other suitable components such as, for example, catalysts, (light-sensitive) initiators, stabilizers, co-reacting monomers and surface-active compounds. It is alternatively possible to add, for example, a quantity of up to 50% by weight of a nonpolymerizable liquid-crystalline material to adapt the optical properties of the material.
Suitable compounds which can be cured in the oriented state, U.S. Pat. No. 4,758,447. In the application described although, no helicoidal order is pursued. A method of manufacturing suitable compounds is described in European Patent Application 0 261 712.
As a rapid curing of the composition is desired, the curing operation is initiated, preferably, by means of actinic radiation. The expression actinic radiation is to be understood to mean herein radiation using light, in particular UV light, X-rays, gamma rays or radiation using high-energy particles such as electrons or ions.